Precisely repositionable bearing cap

ABSTRACT

In a bearing cap of the type which is bolted to a bearing support structure so as to define a bearing bore between the cap and the structure and in which bolt holes for securing the cap to the structure extend through feet of the cap and into the structure, the cap having at least two of the feet, one foot on each side of the bore with at least one bolt hole extending through each foot, the improvement wherein: 
     the cap is sintered powder metal and has an integral boss protruding from the foot around the bolt hole.

This application claims benefit of Provisional Application 60/016,852filed May 3, 1996.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to bearing blocks of the type in which a bearingsupporting bore is formed with half of the bore in a support structureand the other half in a bearing cap which is secured to the supportstructure, in which the cap must be precisely refitted to the supportstructure after machining operations on the bore are carried out. Inparticular, this invention relates to a sintered powder metal bearingcap having bosses for providing precise relocation of the cap relativeto the support structure.

2. Discussion of the Prior Art

The essential function of a bearing cap is to retain and locate a rotaryshaft, or a bearing for a rotary shaft which in turn retains and locatesthe shaft, relative to a support structure. For example, the mainbearing cap of an engine bolts to a bulkhead of the engine crankcase andtogether with the bulkhead retains and locates the crankshaft journal inplace while the crankshaft is rotating. The crankshaft journal runsagainst two half shell bearings which are fitted to the main bearing capand the engine bulkhead semi-circular bores, respectively.

In this case, for vibration free, low friction and quiet running, theroundness of the bore produced by the main bearing cap and the bulkheadis very important. This roundness is achieved by a machining operationcalled line boring. The main bearing caps are bolted to the bulkheads ofthe engine block, and then a boring bar fitted with a cutting tool isused to machine the bores in the assembly. This ensures the two halfrounds formed by the main bearing cap and the bearing block form as nearto a perfect circle as possible. A finishing operation involving agrinding hone is often used to achieve the extremely fine tolerancesneeded for quiet running and efficient engine performance.

However, to install the crankshaft, it is necessary to remove the mainbearing caps from the engine block. After the crankshaft is put inplace, it is necessary to reposition the main bearing caps to thebulkhead so that they are replaced in the identical position theyoccupied during the line boring operation. Any deviation from thatoriginal position produces an out-of-round condition that, in turn,leads to vibration, noise and possibly stiff, high friction crankshaftoperation.

There are a number of conventional structures for re-locating andattaching the main bearing caps to bulkheads when installing thecrankshaft. One such structure is shown in FIG. 1. In this instance, themain bearing cap C has a very precisely machined, snap-width W, which isthe distance across the long axis of the main bearing cap across thefoot sections T of the bearing cap. Similarly, a precision channel P ismachined in the engine block bulkhead B to produce a controlledinterference fit with the feet T when the main bearing cap C is refittedafter crankshaft installation.

This method does not, however, provide relocation in the fore and aftdirection (i.e., in the direction of the axis of the journal bore J).The bolt holes H themselves are used to control the axial repositioning,and since there is a substantial clearance between the bolts F and thebolt holes H of the main bearing cap C, this relocation accuracy isgenerally poor.

In addition, the interference fit between the main bearing caps C andthe channel P in the engine block B in this structure is a variablewhich affects the final roundness of the bore J after re-installation. Ahighly stressed main bearing cap C may stress relieve during engineoperation, thereby changing the roundness of the bore. Also, theprecision machining operations required on the main bearing caps C todefine the snap width W and on the block B to form the channel P, so asto avoid an overstressed or loose main bearing cap in this structure,are relatively expensive.

Another known method of location and attachment is shown in FIG. 2. Thisinvolves the use of hollow dowels D. These dowels D are pressed intocounter-bored holes L in the engine block bulkhead P. The dowels D thenlocate in precisely machined counterbores M in the corresponding mainbearing cap foot sections T. The accuracy of installation of the hollowdowels D is dependent upon the precision counterboring of the engineblock and the main bearing cap. Both of these operations have a finitetolerance which, when stacked up with the tolerance on the dowel D outerdiameter, can produce an unacceptable variation in location of the mainbearing cap C. Additionally, this procedure has the added expense ofpurchasing precision hollow dowels, their handling and installation, andthe costly machining of precision bores L in the bulkhead B and M in themain bearing caps C.

In many cases where hollow dowels as shown in FIG. 2 are used, theengine block channel/main bearing cap snap width relocation method ofFIG. 1 is also used. This combination is expensive and, in fact, canproduce a situation where the interference fits between the snap-widthand channel are in conflict with the interference fits between thehollow dowels and the main bearing cap or bulkhead holes.

SUMMARY OF THE INVENTION

The current invention provides a new and better, but less expensive, wayof precisely relocating a bearing cap relative to the bulkhead of abearing block for reattachment after machining. A bearing cap of theinvention is of the type which is bolted to a bearing support structureso as to define a bearing bore between the cap and the structure and inwhich bolt holes for securing the cap to the structure extend throughfeet of the cap and into the structure. As with prior art bearing caps,a cap of the invention has at least two feet, one on each side of thebore with one or more bolt holes extending through each foot. However, abearing cap of the invention is sintered powder metal and has anintegral boss protruding from each foot around the bolt hole. The bossfits into a counterbore formed in the bulkhead around the bolt hole andis of a shape and ductility so that the counterbore and the bossplastically conform to one another, so that when the bearing cap isremoved from the bulkhead and subsequently refitted, it is preciselylocated relative to the bulkhead by the preformed indentations formedbetween the boss and counterbore when the bearing cap was first fittedto the bulkhead.

In this aspect, the boss is preferably tapered and may be provided withaxial splines which either conform to the counterbore if the bulkhead isa relatively hard material such as cast iron, or bite into the bulkheadif the bulkhead is relatively soft, such as if it is an aluminum alloy.A lead-in radius may be provided on a leading edge of the boss to helpinitially locate the boss in the bulkhead counterbore.

Plastic conformance between the bulkhead counterbore and the boss isfacilitated by the boss and remainder of the bearing cap being sinteredpowder metal, which is not fully dense. However, it may also need to beductile, depending on the material of the bulkhead, and if so it ispreferably a liquid phase sintering powder metal material. Such amaterial preferably is a powder metal alloy of iron containingphosphorus from ferrophosphorus powder with a phosphorus content of 0.4to 0.7% and a carbon content of 0 to 0.8%. Additional strength may beachieved with the addition of copper in the amount of 0 to 4% withoutloss of ductility.

In another preferred aspect, a moat is formed in each foot around atrailing end of the boss. The moat creates a void into which material ofthe bulkhead may bulge or expand when it is deformed by the insertion ofthe boss.

In another aspect, the boss may be oblong in the axial direction of thebore in the bearing cap, so as to provide an interference fit with thecounterbore in that direction. The snap width of the bearing capprovides an interference fit in the lateral direction, so that togetherthe boss and snap width accurately locate the bearing cap in alldirections, without the interference fit of the boss significantlyinterfering with the interference fit of the snap width.

In another form, depressions can be formed in the faces of the feet, soas to increase the clamping pressure for a given bolt loading at theinterface between the faces of the feet and the engine block bulkhead.

Other objects and advantages of the invention will be apparent from thedetailed description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a prior art main bearing cap securedto an engine bulkhead;

FIG. 2 is a cross-sectional view of another prior art main bearing capsecured to an engine bulkhead;

FIG. 3 is a side elevation view of a main bearing cap incorporating theinvention;

FIG. 4 is a bottom plan view of the main bearing cap of FIG. 3;

FIG. 5 is a fragmentary detail side elevation view of a foot of the mainbearing cap of FIGS. 3 and 4;

FIG. 6 is a fragmentary bottom plan view of the foot of FIG. 5;

FIG. 7 is a view similar to FIG. 5 but of an alternate embodiment;

FIG. 8 is a bottom plan view of the foot of FIG. 7;

FIG. 9 is an enlarged fragmentary detail bottom plan view of the foot ofFIG. 8;

FIG. 10 is a partial cross-sectional view as viewed from the plane ofthe line 10--10 of FIG. 9;

FIG. 11 is a partial cross-sectional view as viewed from the plane ofthe line 11--11 of FIG. 9;

FIG. 12 is a partial cross-sectional view as viewed from the plane ofthe line 12--12 of FIG. 11;

FIG. 13 is a partial cross-sectional view as viewed from the plane ofthe line 13--13 of FIG. 11;

FIG. 14 is a view similar to FIG. 5 but of another alternate embodimentof a foot for a bearing cap of the invention;

FIG. 15 is a bottom plan view of the foot of FIG. 14;

FIG. 16 is a view similar to FIG. 5 but of another alternate embodimentof a foot for a bearing cap of the invention;

FIG. 17 is a bottom plan view of the foot of FIG. 16;

FIG. 18 is a view similar to FIG. 5 but of another alternate embodimentof a foot for a bearing cap of the invention;

FIG. 19 is a bottom plan view of the foot of FIG. 18;

FIG. 20 is a side elevation view of another alternate embodiment of abearing cap of the invention, similar to FIG. 3;

FIG. 21 is a bottom plan view of the bearing cap of FIG. 20;

FIG. 22 is a detail bottom plan view of the left foot shown in FIGS. 20and 21;

FIG. 23 is a detail side elevation view of the foot shown in FIG. 22;

FIG. 24 is a view of how a bearing cap can be loaded in operation;

FIG. 25 is a bottom plan view of another alternate embodiment of abearing cap of the invention; and

FIG. 26 is a bottom plan view of another alternate embodiment of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 3 and 4 illustrate a main bearing cap 10 of the invention. The cap10 defines a semicircular bore 12 which together with the semicircularbore of the engine bulkhead (see, for example, FIG. 2) defines the boreJ (FIG. 2) through which the crankshaft of the engine extends and isjournaled. Journal bearings may be received in the bore between thesurface of the crankshaft and the surface of the bore J, as is wellknown. Cap 10 may be notched as at 14 to receive an ear of the journalbearings so as to prevent the journal bearings from rotating relative tothe cap 10 and bulkhead B. The semicircular bore 12 extends through thebearing cap 10 from the front side 16 to the rear side 18.

The bore 12 defines on each of its lateral sides a foot portion 22 ofthe cap 10. A bridge portion 24 joins the two foot portions 22. A bolthole 26 extends through each foot portion 22 from the top side 32 to thebottom 34 of the cap 10. The cap 10 may also be provided with threadedset screw holes 36 extending from the lateral sides 38 and 39 at rightangles into the respective bolt holes 26 so as to lock the retainingbolts (F in FIG. 2) in position after the cap 10 is bolted to the enginebulkhead (B in FIG. 2) support structure.

Projecting from the bottom side 34 of each foot 22 around the respectivebolt hole 26 is a boss 40. Each bolt hole 26 extends through itscorresponding boss 40. FIGS. 5 and 6 show in detail the structure of theboss 40. The two bosses 40 are identical, so only one will be describedin detail.

The boss 40 extends for 360° around the bolt hole 26 and is itselfsurrounded by a recess or moat 44 which is formed in the bottom surface34 of the foot 22 for the purpose described below.

The bolt hole 26 extends into the engine bulkhead B where it is threadedso that bolts F, as shown in FIG. 2, may be used to secure the cap 10 tothe bulkhead B. The bulkhead bolt holes are also counterbored, as shownat L in FIG. 2, so as to receive the bosses 40 in the counterbores ofthe bulkhead. However, the counterbores L of the bulkhead need not be asprecise in diameter or position as was necessary when using theprecision hollow dowels D as shown in FIG. 2, because the boss 40 istapered and the boss 40 and counterbore L are conformable to oneanother.

To effect perfect mating of the parts during line boring andsubsequently thereafter when the crankshaft is installed, the mainbearing cap 10 is made by sintered powder metallurgy, with the bosses 40molded integrally with the feet 22 and remainder of the bearing cap 10.As shown in FIGS. 5 and 6, the boss 40 tapers from a minor diameter atits leading edge 46 to a larger, major diameter at its trailing edge 48.The minor diameter is chosen to be less than the diameter of thecounterbore L in the bulkhead B, and the major diameter is chosen to beequal to or slightly greater than the diameter of the counterbore L.This tapering of the boss 40 ensures that the main bearing cap 10 is inthe identical position after crankshaft installation as it was when itwas line bored. The angle of the taper is preferably greater than 70 soas to ensure easy removal of the bearing cap 10 from the bulkhead afterline boring.

An alternate embodiment of the boss 40, designated 140, is shown inFIGS. 7 and 8, with details shown in FIGS. 9-13. The boss 140 isidentical to the boss 40, except as shown and described below. The boss140 shown in FIGS. 7 and 8 has linear splines 160 angularly spaced apartall the way around its circumference. Leading edge 146 of the boss 140defines the minor diameter of the boss 140, which is less than thediameter of the counterbore in the bulkhead into which the boss 140fits, and the boss 140 tapers to its major diameter at its trailing edge148, which is somewhat greater than the counterbore diameter into whichthe boss fits.

As shown in FIGS. 9-13, the linear splines 160 are flat from leadingedge 146 to line 162, which is at approximately the axial midpoint ofthe boss 140, and are pointed and continue to taper outwardly at a moreshallow angle from the midpoint 162 to the trailing edge 148. Theunderlying tubular body 164 of the boss 140 may also taper from leadingedge 146 to midpoint 162 and may at that point become constant indiameter to the trailing edge 148 so as to provide adequate support tothe splines 160.

FIGS. 14-19 show other alternate embodiments of the invention. Elementscorresponding to elements of the boss 140 are labeled with the samereference numeral plus 100 for FIGS. 14 and 15, plus 200 for FIGS. 16and 17 and plus 300 for FIGS. 18 and 19.

The boss 240 shown in FIGS. 14 and 15 is identical to the boss 140,except that it is not provided with axially running linear splines 160.The boss 340 shown in FIGS. 16 and 17 is identical to the boss 40 ofFIGS. 3-6, except that it does not extend for 360° around the bolt hole26. The moat 344 is also coterminous with the trailing edge 348 of theboss 340. The boss 440 is the same as the boss 40, except that it isprovided with ribs or axially running linear splines 460 which are flatfrom their leading edges to their trailing edges.

The exact design of the boss used for practicing the invention willdepend upon the application. There must be sufficient conformancebetween the bosses 40 and the counterbores L of the supporting structureso as to precisely locate the bearing cap 10 relative to the supportstructure. If additional conformance is needed, a design utilizing thelinear splines such as 160 or 460 may be used. The combination of theselinear splines and the fact that the sintered powder metal is not fullydense, results in the needed conformance between the boss and thecorresponding bulkhead counterbore.

Where the bulkhead material is an aluminum alloy, for example, thelinear splines bite into the softer counterbore to make a perfect fit.Any bulging of the aluminum is accommodated by the moat 44, 144, 244,344, or 444. In the case of a cast iron bulkhead, which is relativelyhard and non-conforming, the splines can condense and conform to thecast iron counterbore, and, again, form a perfect fit.

FIGS. 20-23 illustrate another alternate embodiment of a bearing cap ofthe invention. Elements corresponding to elements of the boss 140 arelabeled with the same reference numeral plus 400.

The boss 540 is the same as the boss 140, except that it is oblong(which includes oval), having its longer dimension in the direction ofthe crankshaft which is retained by the bearing cap, i.e., in the axialdirection of the bore 412. The result is that the bosses 540 engagetheir round engine block bulkhead counterbores in such a way as toprevent relative motion in the axial direction but provide a clearancein the lateral direction, which is the direction that the snap width(between surfaces 438 and 439) provides for location. Thereby, by theoblong bosses 540 providing an interference fit in the axial directionand the snap width providing an interference fit in the lateraldirection, the bearing cap 410 is accurately located in all directions.

Since the boss 540 is oblong, the recess or moat 544, which has a roundouter periphery, varies in width as illustrated. The hole 526 is atruncated round shape, having its round shape truncated by laterallyextending flats which are spaced far enough apart in the axial directionto permit passage of the bolt F for securing the cap 510. This shapeallows substantial clearance with the

In FIGS. 20 and 21, a 360° boss 540 is shown on the left side and a boss540 is shown on the right which extends for less than 360°, extendingfor approximately 270° with its inwardmost quadrant absent. The moat 544of the right boss 540 is also truncated. It should be understood thatthe bosses can be different as shown, or can be the same, with bothbeing 360° or 270° bosses.

The precise installation of the main bearing cap 10, 110, 210, 310, 410or 510 with any of the bosses described above can be achieved bytightening the retaining bolts F alone, or alternatively, by applyingindependent pressure to the assembly, for example, from a hydraulic ram.After line boring, the bearing cap is readily removed due to the taperedgeometry of the installation interface. After installing the crankshaft,the bearing caps are replaced, and the integral bosses nest into theirpreformed positions (preformed when the cap was initially mounted to thesupport structure prior to line boring) with great accuracy.

As stated above, the particular design of the boss will depend on theapplication. The principal variables in the design are the taper angle,the length of the boss, the relative lengths of the tapered and straightportions of the boss, the number, width, and radial height of anyvertical splines, and the radial wall thickness of the boss. The leadingedge of the splines may be tapered at a higher angle as shown in FIG. 10or may have a small lead-in radius as shown in FIG. 18 to aid in initiallocation of the bearing cap bosses into the bulkhead counterbores. Theparticular design of a bearing cap incorporating the invention willdepend upon various specific design details of the bulkhead, such aswhether a bearing notch is needed in the cap, wall thicknesses neededbetween the bolt hole and the side of the bearing cap, the material ofthe bulkhead, and the design of the bulkhead counterbore hole, forexample, with respect to lead-in chamfers or even a preformed taper. Inall cases, however, it is essential that the sintered powder metalbearing cap boss produce a mating surface to ensure identical relocationafter installation of the crankshaft, by plastically conforming to thecounterbore, causing the counterbore to plastically conform to the boss,or a combination of both.

As mentioned above, for practicing the invention, the bearing cap mustbe made of sintered powder metal. A desirable quality of the powdermetal material of the bearing cap for carrying out the invention isductility. Since the splines, or the body in some cases, will yieldplastically to some extent during the initial installation process, itis important to avoid cracking. Most powder metal ferrous materials areinherently brittle. To overcome this potential difficulty, it ispreferable to use a material that has an adequate ductility.

There are a number of ways of improving the ductility of sintered powdermetal ferrous materials, but most of them are expensive or inapplicableto bearing caps. However, an appropriate liquid phase sintering systemis particularly useful in providing the necessary ductility in thisapplication. An example of this process involves the use of a phosphoruscompound such as ferrophosphorus. A small amount of ferrophosphoruspowder is added to the ferrous material powder during powder blending.After compaction and during the thermal treatment stage (sintering),this small amount of ferrophosphorus becomes molten and dramaticallyincreases the rate of atomic diffusion during the sintering process.This enhanced diffusion produces a rounding of the microporosity in thesintered powder metal component which, in turn, provides increasedductility.

To achieve this, the composition of the powder metal material from whichthe bearing cap of the invention is made should contain 0.4 to 0.7%phosphorus (preferably 0.4 to 0.6% phosphorus), a carbon content of 0 to0.8% carbon (preferably 0.4 to 0.6% carbon) and with the balance beingessentially iron (neglecting impurities). This material with thepreferred percentages can produce a tensile elongation of 3%, which isadequate for straight spline conformance to a cast iron counterbore, andalso strong enough to indent a cast aluminum alloy bulkhead. Additionalstrength can be attained by the addition of 0 to 4% copper in the finalmix of the material for making bearing caps of the invention withoutloss of ductility.

In practicing the invention, it is important to ensure dimensionalconsistency of the distance between the axial centers of the bosses. Itis relatively inexpensive to control the counterbore L diameter holecenters in the engine block bulkhead by the use of appropriate drillguides or computer controlled drillheads. However, to control thedistance between the boss centers of bearing caps of the inventionrequires some form of dimensional control during or after the sinteringoperation. One example of an appropriate procedure is to repress thebearing cap in a set of tools which will straighten and adjust thedimensions of the component. This is a procedure well known in thepowder metallurgy industry as repressing (also known as sizing orcoining). An alternative approach is to use a fixture which locates andretains the bearing cap holes in position during sintering. Such afixture could be made from either stainless steel or molybdenum and mayconsist of a U-shaped staple like structure, the legs of which areinserted into the bolt holes of the main bearing cap, thereby avoidingdistortion during the sintering operation.

A common problem encountered in main bearing cap joints is "fretting".This is the relative micromovement of the clamped contact surfaces ofthe bearing cap and bulkhead at high frequency that results in damage tothe surfaces. Fretting fatigue is a possible outcome of this mechanism.

When a main bearing cap is constrained laterally in the block by a snapwidth channel as shown in FIG. 1, it can still move fore and aft(axially) and also from side to side (laterally) under load. Fore andaft motion is due to crankshaft bending (especially in V- engines) whichcauses a rocking motion. Since there is no restraint in this directionother than bolt clamp pressure, microsliding, and therefore fretting,can occur. Similarly, as illustrated in FIG. 24, when the crankshaftloading X is pushing the cap to the "right", the left foot is pulledaway from the snap channel as indicated by arrows Y to create aclearance at the area indicated by the arrow Z.

The present invention, which provides an integral hollow dowel on thebearing cap foot, improves this situation since the dowel serves to fixthe position of the foot relative to the block. The fretting problem canbe further mitigated by hollowing out the footprint of the bearing cap,which has the effect of raising the clamping pressure for a given boltloading. By appropriate geometry, the remaining metal forms a land thatincreases the quality of clamping close to the bearing shell.

The technique of reducing area to raise clamping pressure is not new.However, it is very costly to achieve in volume production. The currentpredominant process of making bearing caps is by casting and machining.To machine precision hollow forms in a casting is prohibitivelyexpensive. Using powder metallurgy, however, hollows can be molded intothe foot with great precision for no extra cost beyond the initialtooling face form costs. Examples of four suitable forms for producingthe indicated void areas V1-V4 (approximately 0.010-0.020 inches deep)and corresponding planar contact areas A1-A4 are shown in FIGS. 25 and26. These voids may be used either with or without integral bosses asdescribed above and may be used in any combination. Experimentation withpressure sensitive paper and finite element analysis simulation showsthat the hollowed out foot surface raises the clamping pressure by thefollowing percentages (the void area given is for each void and thereare two voids per foot as illustrated):

    ______________________________________                                                                   Clamping                                                                      Load                                               Contact Area (in.sup.2)                                                                       Void Area (in.sup.2)                                                                     Increase                                           ______________________________________                                        A1 = 1.0957     V1 = .2942 32%                                                A2 = 1.1373     V2 = .2936 33%                                                A3 = 1.0191     V3 = .2936 30%                                                A4 = 1.0504     V4 = .3159 33%                                                ______________________________________                                    

Many modifications and variations to the preferred embodiment asdescribed will be apparent to those skilled in the art. Therefore, theinvention should not be limited to the embodiments described but shouldbe defined by the claims which follow.

We claim:
 1. In a bearing cap of the type which is bolted to a bearingsupport structure so as to define a bearing bore between said cap andsaid structure and in which bolt holes for securing said cap to saidstructure extend through feet of said cap and into said structure, saidcap having at least two of said feet, one said foot on each side of saidbore with at least one bolt hole extending through each said foot, theimprovement wherein:said cap is sintered powder metal and has anintegral boss protruding from said foot around said bolt hole.
 2. Theimprovement of claim 1, wherein said boss is tapered.
 3. The improvementof claim 1, wherein axial splines are provided on the outside of saidboss.
 4. The improvement of claim 1, wherein a lead-in radius isprovided on a leading edge of said boss.
 5. The improvement of claim 1,wherein said bearing cap is made of a liquid phase sintering powdermetal material.
 6. The improvement of claim 5, wherein said material isa powder metal alloy of iron containing phosphorus from ferrophosphoruspowder.
 7. The improvement of claim 6, wherein said material has aphosphorus content of 0.4 to 0.7% and a carbon content of 0 to 0.8%. 8.The improvement of claim 7, wherein said material has a copper contentof 0 to 4%.
 9. The improvement of claim 1, wherein a moat is formed ineach said foot around a trailing end of said boss.
 10. The improvementof claim 1, wherein said boss is oblong with a major axis in an axialdirection relative to said bearing bore.
 11. The improvement of claim 1,wherein each said foot has a planar surface which abuts a planar surfaceof said bearing support structure, and wherein a void is formed in saidplanar surface.
 12. In a bearing block of the type in which a bearingcap is bolted to a bearing support structure so as to define a bearingbore between said cap and said structure and in which bolt holes forsecuring said cap to said structure extend through feet of said cap andinto said structure, said cap having at least two of said feet, one saidfoot on each side of said bore with a bolt hole extending through eachsaid foot, the improvement wherein:said cap is sintered powder metal andhas an integral boss protruding from said foot around said bolt hole;said support structure has a counterbore around said bolt hole forreceiving said boss; and said boss and said support structureplastically conform to one another when said boss is seated in saidcounterbore.